A typical prior art solid oxide fuel cell system is fueled by a hydrogen-containing reformate fuel derived from catalytic reforming of either liquid or gaseous hydrocarbons such as gasoline or methane. Thus, such a fuel cell system must have a source of supply, which may be onboard storage, of a hydrocarbon fuel. Solid oxide fuel cell stacks and reformers operate at temperatures elevated well above ambient, for example, in the range between about 550° C. and about 850° C., or even higher.
Fuel cell systems are used, or contemplated for use, as auxiliary power units (APUs) for providing electric power in, for example, vehicles wherein motive power is derived from another, primary source such as an internal combustion engine. Such systems are also contemplated as stand-alone electric generators, which may define combined heat and power (CHP) systems. For discussion purposes herein, all such contemplated uses of fuel cell systems are referred to as APUs.
A first recognized problem in operation of prior art fuel cell systems is the time required to change over from a non-productive cold start mode to an operating mode productive of electricity. It is known to form and ignite a combustible hydrocarbon mixture and to pass the hot combustion gases through the reformer and the fuel cell stack to bring those components up to operating temperature in from about 20 minutes to up to several hours. Drawbacks of this procedure are that it is wasteful of fuel, and it creates undesirably large thermal stresses on elements of the reformer and stack which can damage or destroy parts of the system.
A second recognized problem is how to maintain a prior art fuel cell system at a standby readiness condition (that is, at near-operating temperature) such that the system may be changed over to operating mode in a very short period of time. It is known to highly insulate the reformer and stack to minimize heat loss during standby mode and to continue periodic combustion of small amounts of hydrocarbon fuel to keep the reformer and stack warm on standby. Again, this procedure is wasteful of fuel.
The start-up and standby losses can be mitigated in large utility or industrial systems by using the system in more of a base-load where there is almost always sufficient electrical power generated for the system to be thermally self-sustaining, or by adding additional or premium insulation. In small scale systems, however, a self-sustaining usage profile may not be economic when compared to purchasing lower cost grid electricity most of the time, and additional or premium insulation may not be acceptable in terms of packaging (size of the unit) and cost.
If by-product thermal energy is needed, such as to keep a vehicle warm when parked in winter conditions, or to heat or provide hot water for a building, then the combustion of a continuous or periodic quantity of fuel inside the SOFC system can serve to regulate the temperature of the stack, reformer catalyst, and other “hot zone” components to a partially or fully warmed up condition and to supply hot exhaust for the external thermal load. However, where “waste” heat is not needed, or where heating can be performed at lower cost or with less generation of greenhouse gases, it would be preferable not to burn fuel in the SOFC to keep the system at stand-by. For example, off-peak electricity supplied by renewable, nuclear, or high efficiency power plants, coupled with an electrically powered heat pump system may be more efficient for space heating in a building than keeping the SOFC hot for long periods of time for its thermal by-product only.
What is needed in the art is an alternative method and apparatus for warming up or maintaining an SOFC hot zone at a stand-by temperature consuming little or no hydrocarbon fuel specifically for such heating.
It is a principal object of the present invention to warm up or maintain an SOFC hot zone at a stand-by temperature without consuming hydrocarbon fuel within the SOFC.